We know that blackbody radiation produces an
intensity-frequency curve that is dependent on temperature. The idea of
course is that you are looking into the cavity through a hole. Why isn't
the outside of the blackbody also the same color as the inside (for a
given temperature)? And what it we had a blackbody made of sodium.
Wouldn't there be a peak in frequencies emitted that corresponds to the
traditional sodium spectrum? Is then the blackbody curve
A black body is an idealized thing -- a hypothetical object whose
radiation is perfectly described by certain simple equations derived by
some guys (Stefan and Boltzmann) who were thinking about how individual
molecules behave without paying attention to the details of those
molecules. All objects can be approximated as black bodies. For some
objects in some circumstances the approximation is good enough that you
don't need to describe them further; for other objects, the
approximation is so bad it's useless unless combined with other
approximations that idealize radiation in other ways.
A true description of an object's radiation -- one that would be
correct in every detail -- would be so complicated you'd never get it
all written down, so you'd never be able to use it to answer simple
practical questions like "If it's red hot, will it melt aluminum?".
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Update: June 2012